1. Field of the Invention
The present invention concerns a device for determining the position of a rotor of a rotary electrical machine comprising a stator.
The invention finds a particularly advantageous application in the field of reversible machines, or alternator/starters, used in the automotive industry, both in alternator mode and in motor mode on starting up or as an assistance to takeoff (boosting) as from 500 revolutions/minute.
2. Description of the Related Art
A reversible machine comprises an alternator including:                a rotor comprising a field winding associated conventionally with two collecting rings and two brushes by means of which an excitation current is brought;        a multiphase stator carrying several coils or windings that are connected in star or delta in the most frequent case of a three-phase structure and that deliver to a bridge rectifier, in alternator functioning, a converted electric power. The machine has two bearings, front and rear, for fixing it to the thermal engine and for fixing the stator. The stator surrounds the rotor. The rotor is carried by a shaft supported by the front and rear bearings. The brushes are connected to a regulator of the alternator in order to maintain the voltage of the alternator at the required voltage for a battery depending on whether it is offload or under charge.        
The alternator converts a rotation movement of the rotor driven by the thermal engine of the vehicle into an electric current induced in the stator windings.
The alternator may also be reversible and compose an electric motor, or rotary electrical machine, making it possible to drive in rotation, via the rotary shaft, the thermal engine of the vehicle. This reversible alternator is called an alternator/starter. It converts mechanical energy into electrical energy and vice versa.
Thus, in alternator mode, the alternator/starter charges in particular the vehicle battery and consumers whilst in starter mode the alternator/starter drives the thermal engine, also referred to as the internal combustion engine, of the motor vehicle in order to start it.
In reversible machines in the automotive industry, for example, functioning according to motor or starter modes, the stator must be controlled for current so as to at all times apply to the rotor the torque necessary both for starting it up and to impart to it the required rotation for the functioning of the motor. However, this torque to be applied to the rotor, and therefore the current to be supplied to the phases of the stator, is a sinusoidal function of the position, referenced by the angle θ, of the rotor with respect to the stator, and hence the need to determine this position precisely.
To this end, certain reversible electrical machines, in particular those used in the automotive industry, are currently equipped with a device known by the term resolver disposed at the end of the shaft of the rotor of the machine. Such a resolver is described in U.S. Patent Publication No. 2002/0063491 A1. It has itself a stator and a rotor that are respectively fixed with respect to the stator and rotor of the machine. The resolver measurers the magnetic field issuing from its own rotor. This magnetic field, being fixed with respect to the rotor, which is itself fixed with respect to the rotor of the machine, represents the position of the actual rotor of the machine.
However, this type of equipment has a certain number of drawbacks. Resolvers are in fact expensive and their implementation to make them operational is complex because of the coupling to be effected between such a resolver and the machine itself requiring the presence of electronic computing components to supply a correct position of the rotor of the machine from the coupling parameters.
In addition, they are sensitive to the magnetic disturbance caused by the stray magnetic field produced by the rotor, which causes a malfunctioning of the system and therefore measurement errors and poor control of the machine. To limit this drawback, it is necessary to have recourse to magnetic protection, such as a stainless-steel tube placed between the rotor and the resolver at the shaft end. In addition, the mechanical strength of this device is not perfect since it is particularly sensitive to the vibration of the machine because of the mounting on the end of the rotor shaft.
Finally, their resistance to salt spray and dust is not completely satisfactory.